A common building heating system utilizes a series of electrical heaters around the periphery of the building to compensate for heat losses through the exterior building walls. These heaters may be supplemented by heating of air circulated through the building interior, particularly in office buildings, with thermostatic control of the air circulation system used to determine room temperatures. In any such system, changes in external environmental factors such as accumulation or dispersion of clouds, increase or decrease in wind velocity or change in wind direction, temperature variation, etc., cause appreciable changes in the rate of heat loss (or gain) through the walls, with a corresponding need for change in energization of the heaters. Moreover, heat flow conditions through each building wall are likely to be different from the other walls. Thus, at midday on a cold, sunny winter day the north wall of a building may exhibit a high rate of heat loss, the east and west walls may show moderate heat loss rates, and the south wall may exhibit a very low heat loss rate or even a net heat gain from the exterior to the interior due to solar load.
Various control systems have been proposed for peripheral electrical heating systems of this kind, intended to match the output of the heaters with the heat rate transfer through the building walls. One of the more successful systems is that described in Kralovec U.S. Pat. No. 3,450,862, which, in its most sophisticated version, includes an external solar heat sensor, a shielded outside thermal sensor intended to be independent of solar conditions, an outside temperature sensor, indoor thermostats, and an internal thermal sensor that monitors output of the heaters to afford a feedback for the control. That system, however, is subject to operational problems due to accumulation of snow or ice on any of the outside sensors or to physical damage to those sensors. A change in the environment (e.g., erection or removal of an adjacent building) can upset the control system calibration. And installation cost is high due to the need for connections through the building walls or roof and additional required wiring.
Another control for peripheral building heating systems, described in Rall et al U.S. Pat. No. 4,274,475, eliminates some of the aforementioned problems because it eliminates the outside sensors in favor of a thermopile differential sensor mounted on the inside of the building wall, often on a window. But the Rall et al arrangement requires mounting of the thermopile sensor in good heat exchange relation on the wall, a relationship not easily achieved and difficult to maintain over extended periods of time. And that control is likely to suffer from excessive hunting through failure to maintain effective monitoring of actual heater performance.